linux-kernel - Re: [PATCH v1 0/2] x86/smp: Fix power regression introduced by commit 96040f7273e2

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Message-ID: <20250528131759.GA39944@noisy.programming.kicks-ass.net>
Date: Wed, 28 May 2025 15:17:59 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc: x86 Maintainers <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Linux PM <linux-pm@...r.kernel.org>, Len Brown <lenb@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...e.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Todd Brandt <todd.e.brandt@...ux.intel.com>
Subject: Re: [PATCH v1 0/2] x86/smp: Fix power regression introduced by
 commit 96040f7273e2

On Wed, May 28, 2025 at 02:53:13PM +0200, Rafael J. Wysocki wrote:
> Hi Everyone,
> 
> Commit 96040f7273e2 ("x86/smp: Eliminate mwait_play_dead_cpuid_hint()")
> that shipped in 6.15 introduced a nasty power regression on systems that
> start with "nosmt" in the kernel command line which prevents it from entering
> deep package idle states (for instance, PC10) later on.  Idle power, including
> power in suspend-to-idle, goes up significantly on those systems as a result.
> 
> Address this by reverting commit 96040f7273e2 (patch [1/2]) and using a
> different approach, which is to retain mwait_play_dead_cpuid_hint() and
> still prefer it to hlt_play_dead() in case it is needed when cpuidle is
> not available, but prefer cpuidle_play_dead() to it by default (patch [2/2]).

I don't understand. The revert says the reason it regresses is that it
goes into play_dead before cpuidle is initialized. The fix is then to
call cpuidle first.

But if cpuidle isn't initialized yet, how does that fix anything?